Orthogonal Frequency Division Multiplexing (OFDM) is a proven access technique for efficient user and data multiplexing in the frequency domain. One example of a system employing OFDM is Long-Term Evolution (LTE). LTE is the next step in cellular Third-Generation (3G) systems, which basically represents an evolution of previous mobile communications standards such as Universal Mobile Telecommunication System (UMTS) and Global System for Mobile Communications (GSM). It is a Third Generation Partnership Project (3GPP) standard that provides throughputs up to 50 Mbps in uplink and up to 100 Mbps in downlink. It uses scalable bandwidth from 1.4 to 20 MHz in order to suit the needs of network operators that have different bandwidth allocations.
Recent standardization efforts in 3GPP towards 5G cellular systems also focus on OFDM for so-called New Radio (NR), including support to carrier frequencies up to 100 GHz. NR will support so-called Ultra-Reliable Low-Latency Communications (URLLC), characterized by high reliabilities and low latencies for critical applications such as emergencies, health, or even tactile Internet. Other wireless standards like WiFi (IEEE 802.11), WiMAX (IEEE 802.16) or Digital Video Broadcasting Terrestrial (DVB-T) also employ OFDM.
Among the disadvantages of OFDM, its sensitivity to Doppler, phase noise and frequency offset, as well as its large peak-to-average power ratio (PAPR), are among the hardest challenges to overcome. Large PAPR signals lead to low efficiency of the power amplifiers (PAs), because the PA operating point should be designed well within the linear region hence requiring large back-off values (usually higher than 10 dB, unless clipping techniques are applied). This drawback is aggravated at high frequencies because of the inherently lower power efficiency of radio frequency (RF) hardware above 6 GHz. Sensitivity to frequency misalignments is also critical at high frequencies, as well as the Doppler spread caused by movements of the user and/or the environment, both of them linearly increasing with the carrier frequency. Finally, phase noise introduced by RF oscillators is another issue that can be very significant beyond 6 GHz, hence introducing non-additive impairments at the receiver side that, except for the so-called common phase error (CPE), can be very difficult to overcome.
There is a large body of research studying alternatives to OFDM aimed at overcoming some of the above limitations. In [1], the authors propose a constant-envelope waveform based on modulating the instantaneous frequency of the carrier signal with an OFDM modulating signal. Such waveform is seen to be particularly robust to multipath, phase noise and frequency offsets, but additional robustness to Additive White Gaussian Noise (AWGN) is not yet studied. Critical communications must often ensure very high reliability at the air interface even in low signal-to-noise ratio conditions, hence demanding specialized techniques to overcome noise and interference without compromising performance.
3GPP has also proposed suitable modifications to existing OFDM-based waveforms in so-called NB-IoT (Narrowband IoT) and LTE-M air interfaces, with the goal of improving coverage and addressing traditionally challenging scenarios (like deep indoor environments). However, these air interface variants do not offer good protection against Doppler, phase noise or frequency offsets. Both NB-IoT and LTE-M are based on OFDM waveform hence suffering from the same limitations as to the impact of frequency offset, phase noise and Doppler on performance.
Ongoing standardization of NR in 3GPP is focusing on so-called Phase Tracking Reference Signals (PTRS), aimed at compensating the impact of CPE in OFDM waveforms. Non-constant phase errors are, however, generally not possible to compensate by means of reference signals, and subcarriers are assumed to be wide enough (as per the scalable numerology in NR) so as to make phase noise less harmful. Too wide a subcarrier spacing, however, may lead to non-flat channel conditions at the subcarriers, hence demanding intra-subcarrier equalization in frequency-selective channels, with the subsequent complexity.
In summary, current state of the art cannot provide improved reliability and simultaneous protection against Doppler, phase noise and frequency instability with standard waveforms. More adequate waveforms are therefore required in order to provide sufficient robustness to those impairments, while also overcoming the impact of low power efficiency at the PA.